I would guess that your alternator/voltage regulator has problems and the battery is possibly overcharging , heating up, and venting fluid. I would have the alternator/regulator checked ASAP
My understanding is that the vent tube you are talking about is normally for venting hydrogen gas which may be produced by batteries under certain conditions. My C230 has a vent tube as it is inside the trunk, and H2 gas could get into the car. My 300E has the battery in a "traditional" under hood location, and doesn't use a tube. Instead, it sits on a tray to catch spills.
2003 Firemist Red/grey leather SL 500
2015 Palladium Silver/black mbtex GLK 350
1987 Smoke Silver/burgundy mbtex 300E Sportline (SOLD)
Click to see 87 300E